High-voltage devices or power devices are commonly used as switches or rectifiers in power electronic circuits or in integrated circuits. Some common power devices are the power diode, thyristor, power metal-oxide-semiconductor field-effect transistor (MOSFET), bipolar junction transistor (BJT) and insulated gate bipolar transistor (IGBT). A power diode or MOSFET operates on similar principles to its low-power counterpart, but is able to carry a larger amount of current and typically is able to support a larger reverse-bias voltage in the off-state. High-voltage devices are increasingly made smaller and smaller, reducing dimensions of various features. As the dimensions decrease, the high-voltage devices become increasingly susceptible to interference between adjacent power devices by unwanted cross talk between adjacent devices. In many instances, the cross talk is caused by lateral parasitic substrate current.
The interference between adjacent power devices also manifests as parasitic structure formation forming a latchup circuit between the power devices. A latchup circuit is a type of short circuit with a low-impedance path between parasitic structures. The parasitic structure is usually equivalent to a thyristor, a PNPN structure which acts as a PNP and an NPN transistor stacked next to each other. During a latchup, when one of the transistors is conducting, the other one begins conducting too. They both keep each other in saturation for as long as the structure is forward-biased and some current flows through it. A latchup circuit can cause a product to fail.
Electronic design includes a latchup rule that describes a minimum distance between two devices, sometimes between two isolation structures, to reduce or eliminate the likelihood of a latchup between devices. High-voltage devices use a larger latchup rule. If kept constant, this minimum distance prevents significant size reductions of circuits containing high-voltage devices. Thus, smaller latchup resistant or latchup proof semiconductor structure designs and methods for making the same continue to be sought.